usb: gadget: Revert increase write buffer size

Commit 3fb31996 ("usb: gadget: increase write buffer size") brings
additional memory requirement and leads to memory allocation failure
when trying to allocate buffer to the request in usb_cser_alloc_req.
Hence revert it to fix this.

Change-Id: Iff5d6a2502d7ff094422fc66c20dff13d5b8d4ca
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>